1. Field of the Invention
The present invention relates to a rodless cylinder that is capable of operating smoothly while effectively bearing loads applied in a variety of directions.
2. Description of the Prior Art
Discussion of the Background
Various rodless cylinders have recently found wide use as workpiece feeders in factories or the like.
The rodless cylinders take up a smaller space and can be handled with greater ease than cylinders with rods because the stroke of the rodless cylinders may be shorter. Since dust or foreign matter is prevented from entering rodless cylinders more effectively than cylinders with rods, the rodless cylinders are capable of positioning workpieces with higher accuracy.
U.S. Pat. No. 4,373,427 and German patent No. 3,124,915, for example, disclose rodless cylinders. The rodless cylinder disclosed in German patent No. 3,124,915 has guide grooves defined between an outer circumferential surface of a cylinder tube and an upper surface of a cylinder with a slit defined therein, and guide means engaging in the guide grooves, the guide means being mounted on legs extending from opposite ends of a slide table to side surfaces of the cylinder tube.
More specifically, the rodless cylinder of this type has a cylinder and a piston reciprocally movable in a bore defined in the cylinder, and includes a slide table coupled to the piston and slidable in the longitudinal direction of the piston. The cylinder has a slit defined in its upper surface which provides communication between the bore and the exterior of the cylinder along the direction in which the slide table is movable. The slit is closed by two seal members engaging the piston and the slide table. A resin-made separator is used to separate the seal member engaging the piston from the seal member engaging the slide table. A magnet is attached to an outer circumferential surface o#a piston yoke for detecting the position of the piston when the slide table to which a workpiece is clamped is slidingly displaced along the slit. The magnet is held on the outer circumferential surface of the piston yoke by a magnet holder. The position of the piston is detected when a magnetic field produced by the magnet is detected by a magnetic sensor that is mounted on the outer circumferential surface of the cylinder.
According to the rodless cylinder disclosed in German patent No. 3,124,915, however, the legs from the slide table have to extend outwardly from the outer side surfaces of the cylinder tube for holding the guide means. When the guide means engage the cylinder tube at the time a lateral force is applied, one end surface of the cylinder tube which defines the slit is forcibly brought toward the other end surface of the cylinder tube which also defines the slit. Therefore, the inside diameter of the bore is reduced as a whole, forcing the piston to stop. Therefore, the application of a lateral force causes the piston to stop. When this occurs, the workpiece is unexpectedly brought to a stop, bringing about a highly dangerous condition.
The rodless cylinder has a joint which couples the piston in the cylinder tube to the slide table disposed outside of the cylinder tube, the joint extending into the bore through the slit in the cylinder tube. Thus, upon reciprocating movement of the piston and the slide table in unison with each other, if the slide table is tilted under external forces, an inner wall surface of the cylinder tube and the joint are liable to contact each other. The cylinder tube and the joint are made of a metal such as aluminum or the like. Therefore, when the inner wall surface of the cylinder tube and the joint contact each other and slide against each other, the abrasive sliding engagement tends to wear off metal particles or cause seizure, preventing the slide table from smoothly moving back and forth.
Furthermore, inasmuch as the magnet for detecting the position of the piston is attached to the outer circumferential surface of the piston yoke, the outer circumferential surface of the piston yoke must be machined to provide a recess for holding the magnet. The magnetic sensor for detecting the magnetic field from the magnet is usually disposed on the outer circumferential surface of the cylinder tube. Consequently, the magnet is required to be mounted in the vicinity of the magnetic sensor. Heretofore, it has been necessary to combine the magnetic poles of a pair of magnets for producing a strong magnetic field. For example, the two magnets each having N and S poles are arranged in a slightly staggered relationship to provide a magnetic path of N-S-N-S for generating a strong magnetic field. However, the two magnets may sometimes be arranged to provide a wrong magnetic path of N-S-S-N, for example. If the magnets are placed in the recess in a wrong orientation when the rodless cylinder is assembled, then the magnets fail to produce a desired strong magnetic field.